The present invention generally relates to an image forming apparatus and, more particularly, to an image forming apparatus such as a color printer or a color copy machine for forming a dot image.
A dither method or a tone production method by density pattern is well-known as a method for representing a halftone image by a printer. By using such a method, an image having a screen angle can be formed. Thereby, moire stripes can be eliminated, and uniformity in an average color can be obtained.
However, the dither method or the tone production method by density pattern has a problem in that a resolution in a printed image is decreased as a number of gradation levels is increased by increasing a size of a matrix.
In order to compensate for the above-mentioned drawback, Japanese Laid-Open Patent Application No.3-80768 discloses an image forming apparatus using an image density correcting method using a multi-value dither method. In this method, multi-value image data is corrected by first correcting means, and the corrected multi-value image data is processed by dither processing means. Then, a density characteristic of the multi-value dither data obtained by the dither processing means is corrected by second correcting means in accordance with an output characteristic of a printer. Accordingly, this image forming apparatus can be operated with various printing characteristics.
On the other hand, an image forming apparatus such as a printer must be operate with various pseudo gray scale processes such as dither processes. However, in the above-mentioned conventional image forming apparatus, the density correction is performed by a single xcex3-correction means. Thus, the conventional image forming apparatus cannot perform different pseudo gray scale processes.
It is a general object of the present invention to provide an improved and useful image forming apparatus and method in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an image forming apparatus and method in which pixel data subjected to various dither processes can be corrected with a single correction table.
Another object of the present invention is to provide an image forming apparatus and method in which a reproducibility of a low-density part of a printed image is improved and a density gradation characteristic is stabilized.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an image forming apparatus for forming a dot image having density gradation represented by a plurality of dots, the image forming apparatus comprising:
detecting means for detecting conditions of dots surrounding each dot in the dot image; and
correcting means for correcting pixel data corresponding to each dot in accordance with a difference in a density level of the pixel data between each dot and the dots surrounding each dot.
In the above-mentioned invention, the correcting means may includes means for changing a degree of correction between the dots arranged in the horizontal and vertical directions with respect to each dot and the dots arranged in the diagonal directions with respect to each dot.
Additionally, the correcting means may correct a density level of pixel data corresponding to each dot by using values according to a developing characteristic of the image forming apparatus.
Further, the correcting means may include means for detecting dot information regarding the dots surrounding each dot from level information included in pixel data corresponding to the dots, and for converting the dot information according to previously prepared table information.
Additionally, the correcting means may calculate a correction factor based on dot information with respect to the dots surrounding each dot so as to perform a correction of each dot by using the calculated correction factor.
According to the present invention, a density level of pixel data corresponding to a focused dot is corrected based on a difference in density level between the focused dot and the dots surrounding the focused dot. Thus, an appropriate reproduction of a dot image can be achieved in response to a condition of the dots surrounding the focused dot.
Additionally, a degree of correction is changed between the dots arranged in the horizontal and vertical directions with respect to the focused dot and the dots arranged in the diagonal directions. Thus, for example, a reduction in a thickness of a diagonal line can be compensated for.
Additionally, if the correction factor is calculated after the pixel data corresponding to the dots surrounding the focused dot, an accurate correction can be performed. Further, by performing the correcting operation by using previously prepared table information, a high-speed correcting operation can be achieved.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.